1. Field of the Invention
The present invention relates to a wireless sensor network, and more particularly to a method for generating a superframe structure using beacon scheduling and transmitting data based on the generated superframe structure.
2. Description of the Related Art
A wireless sensor network is a network implemented to collect remote information in a manner differently than that of an existing network implemented for the communication. Such a wireless sensor network includes sensor nodes for processing and transmitting information collected through sensors and sync-nodes for sending transmission information. Since such a wireless sensor network is formed by a number of sensor nodes, the architecture of the sensor nodes must be simply designed. Moreover, since the sensor nodes may be deployed in wilderness areas, it is necessary to design the sensor nodes with low power consummation for operation with their initial battery for many months or many years. Furthermore, mobility must be taken into consideration in the design of the sensor nodes, so that they can be freely moved. In addition, even if some sensor nodes within the network are damaged, it is necessary to implement the architecture so that the damaged nodes have no influence on the network maintenance.
Meanwhile, the IEEE 802.15 Working Group has been defining the standard of the short-range wireless network. Particularly, since the IEEE 802.15.4 standard defined by the IEEE 802.15 Working Group relates to the implementation of short-range wireless network with low power, it is now attracting attentions as a prominent core technology suitable for the application of a sensor network.
A method for transmitting and receiving data in a wireless sensor network based on the IEEE 802.15.4 standard protocol will be briefly described.
FIG. 1A is a flowchart showing a process for transmitting data from a network device to each of a plurality of coordinators (i.e. a PAN coordinator, a first coordinator, and a second coordinator) in a network using beacons. FIG. 1B is a flowchart showing a process for transmitting data from a coordinator to a network device.
First, the process for transmitting data from the network device to each of the coordinators will be described with reference to FIG. 1A. The coordinator within the wireless sensor network periodically generates a beacon for data transmission, and transmits the generated beacon to data transceivers adjacent to the coordinators. Then, the network device having the received beacon performs synchronization with a superframe structure at a proper point of time and transmits its own data frame to the coordinator by using a slotted CSMA-CA The coordinator sends back an acknowledgement (“ACK”) in response to the data reception and reports that it has successfully received the data. Then, the process for transmitting data from the network device to each of the coordinators is terminated.
Next, the process for transmitting data from the coordinator to the network device will be described with reference to FIG. 1B. In this case, the coordinator transmits data by adding information, indicating that it waits to transmit data, to a beacon. After the network device receives the beacon from the coordinator, the network device checks that the coordinator waits to transmit data, and makes a request for data through the slotted CSMA-CA. Then, the coordinator sends back “ACK” in response to the request for the data transmission to the network device, and transmits data through the slotted CSMA-CA. Then, the network device sends back “ACK” in response to the data reception to the coordinator, and reports that it has successfully received data. Then, the process for transmitting data from the coordinator to the network device is terminated.
FIG. 2 is a view showing a tree topology structure of a wireless sensor network based on the IEEE 802.15.4 standard protocol in the prior art.
Referring to FIG. 2, the tree topology structure of the wireless sensor network based on the IEEE 802.15.4 standard protocol includes a PAN coordinator, a first coordinator, a second coordinator, and a network device. The PAN coordinator is disposed as a top layer of the network, and the first coordinator is connected to the PAN coordinator and is a sub-layer of the PAN coordinator. The second coordinator is connected to the first coordinator and is a sub-layer of the first coordinator The network device is connected to each of the coordinators and is a network end device for each of the coordinators.
Further, when the tree topology structure is employed and a beacon is used in the wireless sensor network, synchronization of a superframe through the transmitted beacons transmitted by all of coordinators is maintained. However beacon collision may occur due to beacons simultaneously transmitted from the PAN coordinator and the coordinators within the network. In the end, the coordinators cannot receive intended beacons due to beacon collisions. In order to prevent the beacon collisions, there has been proposed a time division approach and a BOP-based approach.
FIG. 3 is a view showing transmission beacon timing of a coordinator according to a time division approach in the prior art. In the time division approach, for example, an active period including a beacon frame duration and a Superframe Duration (SD) of the PAN coordinator is set to be positioned at an inactive period of the first coordinator for preventing the beacon collisions. However, since the time division approach is a method for randomly positioning an active period and a beacon at an inactive period of a neighboring node, this approach may be efficiently used only if the active period is short and the inactive period is long. In addition, a scope of a selectable inactive period is also reduced according to increase in the number of coordinators in a network. That is, the time division approach has a disadvantage in that it is difficult to avoid beacon collisions according to increases in the distribution density. It also has a disadvantage in that the coordinators receiving the beacons must continually maintain an idle state to receive beacons of neighboring nodes during an inactive period. Therefore, efficiency is remarkably reduced.
FIG. 4 is a view showing beacon transmission timing of a coordinator according to the BOP-based approach in the prior art. The BOP-based approach, which is a contention free scheme, is described with reference to FIG. 4. Each superframe includes a Beacon Only Period (BOP) for beacon transmission. Therefore, each coordinator identifies its own timeslot through the BOP and transmits a beacon only at its assigned timeslot. Further, since each coordinator must receive beacons transmitted from neighboring coordinators, each coordinator waits to receive beacons in a period for beacon transmission by coordinators included in its upper layer. Also, each coordinator must continually wait to receive beacons transmitted from neighboring coordinators during a period subsequent to each coordinator's beacon transmission duration. That is, the coordinator must maintain the active state so that its upper layer coordinators can receive beacons from the starting time of beacon transmission to the end time of the BOP. For example, the first coordinator must be changed into the active state before beacon transmission of the PAN coordinator, and must maintain the changed state (i.e. active state) until the end of the BOP. Therefore, the coordinator must unnecessarily wait to receive beacons even in a period during which a coordinator does not transmit a beacon. Such unnecessary waiting results in wasteful power consumption. Further, if the BOP is separately set within a Superframe Duration (SD), a period for data transmission of the SD may be reduced. Such reduced SD results in the reduction of the network communication efficiency.
In order to solve such problems, there has been requested a plan in which beacons transmitted by coordinators within a network are set to avoid their collisions and consumed power of network administration can be reduced. There has been also requested a plan for increasing network communication efficiency.